1. Field of the Invention
The present invention relates to a method and apparatus for use in processing a substrate. More particularly, the invention relates to a shadow frame for aligning and stabilizing a substrate during processing.
2. Background of the Related Art
In the fabrication of flat panel displays, transistors, liquid crystal cells, metal interconnects and other features are formed by depositing and removing multiple layers of conducting, semiconducting and dielectric materials from a glass substrate. Glass substrate processing techniques include plasma-enhanced chemical vapor deposition (PECVD), physical vapor deposition (PVD), etching and the like. Plasma processing is particularly well-suited for the production of flat panel displays because of the relatively lower processing temperatures required to deposit a film and good film quality which should result from using plasma processes.
In general, plasma processing involves positioning a substrate on a support member (often referred to as a susceptor or heater) disposed in a vacuum chamber and striking a plasma adjacent to the upper exposed surface of the substrate. The plasma is formed by introducing one or more process gases into the chamber and exciting the gases with an electrical field to cause dissociation of the gases into charged and neutral particles. A plasma may be produced inductively, e.g., using an inductive RF coil, and/or capacitively, e.g., using parallel plate electrodes, or by using microwave energy.
During processing, the edge and backside of the glass substrate as well as the internal chamber components must be protected from deposition. Typically, a deposition-masking device, or shadow frame, is placed about the periphery of the substrate to prevent processing gases or plasma from reaching the edge and backside of the substrate and to hold the substrate on a support member during processing. The shadow frame may be positioned in the processing chamber above the support member so that when the support member is moved into a raised processing position the shadow frame is picked up and contacts an edge portion of the substrate. As a result, the shadow frame covers several millimeters of the periphery of the upper surface of the substrate, thereby preventing edge and backside deposition on the substrate.
However, while conventional shadow frames may reduce edge and backside deposition on a substrate, the usable area of the substrate is greatly reduced. Typically, shadow frames comprise a lip portion extending over the edge of the substrate. The lip prevents any portion of the masked area of the substrate from receiving deposition, an effect known as edge exclusion. Consequently, each processed substrate includes an unprocessed, unusable portion which reduces the usable surface area on a substrate and results in lower productivity of the processing system.
One attempt to reduce the edge exclusion is the use of finger-type shadow frames. Finger-type shadow frames comprise a plurality of xe2x80x9cfingersxe2x80x9d or tabs extending outwardly from the shadow frame to stabilize a substrate during processing. The fingers are disposed around the edge of a substrate, thereby increasing the amount of usable substrate surface area as compared to the lip-type shadow frame which comprehensively covers the edge of the substrate.
However, finger-type shadow frames exhibit poor control of the plasma during processing. Successful plasma processing requires a uniform plasma density across the entire upper surface of a substrate during processing. Anomalies in the plasma density result in non-uniform deposition of films on the substrate leading to defective devices and thus further reducing the throughput of the processing system. In the case of flat panel display manufacturing, maintaining a uniform plasma at the perimeter of the substrate is particularly difficult due to various components of the vacuum system which can act as energy sinks. In conventional systems using finger-type shadow frames, for example, the shadow frame and the outermost edge of the substrate define a gap provided to prevent arcing which can occur between the shadow frame and the substrate resulting in damage to the substrate. However, the gap exposes the perimeter of the support member to the plasma and provides a ground to drain the plasma constituents. Thus, the plasma density at a perimeter of the substrate is often substantially less than the plasma density over the central portion of the substrate. Because the deposition thickness is related to the plasma uniformity, non-uniform deposition results.
FIGS. 1 and 2 illustrate the non-uniform deposition resulting from anomalies in the plasma density using a finger-type shadow frame. FIGS. 1 and 2 represent processes performed in a conventional processing chamber wherein the plasma experienced drainage at a perimeter portion of the substrate. Referring to FIG. 1, for example, the deposition rate between about 0 mm and 30 mm and between about 690 mm and 720 mm is shown tapering off rapidly indicating the loss of uniformity. Thus, the edge of the substrate constitutes unusable area, thereby decreasing the margin of the process and the productivity of the processing system.
Another problem with flat panel display processing is the detrimental effects of thermal dynamics. During processing, the support member is heated by means of a heating element, such as a resistive coil, or by other methods in order to heat the substrate disposed thereon. Uniform heat conduction between the support member and the substrate are necessary to ensure uniform deposition. Where the thermal profile across the substrate is not uniform, i.e., where the profile exhibits relative hot and cold spots, the deposition of material onto the substrate is non-uniform and results in defective devices. Flat panel displays are particularly susceptible to the detrimental effects of thermal non-uniformity because of the area of the substrates exposed to deposition material as compared to their thicknesses, and because of the differences in the thermal conductivity of the substrates, typically comprising glass, and the support member, typically comprising a metal. In a typical process, the substrate may be about 30-60xc2x0 C. less than the temperature of the support member which may be heated to a temperature between about 250-470xc2x0 C. The support member will typically be more conductive than the glass but also be locally heated to a greater temperature adjacent to the heating element. This leads to localized heating of the glass which, because of its low expansion coefficient and lower thermal coefficient, will expand at the localized hot spots resulting in warping or bowing. Warping is also induced where a cooling gas is supplied to the backside of the substrate. The cooling gas can provide a backside pressure causing the substrate to bow or warp. To ensure good heat conduction and minimize warping, the substrate and the support member must be maintained in close and uniform contact, thus warping and bowing should be avoided.
Current efforts to achieve good thermal conduction between the substrate and the support member include providing a clamping force at the perimeter of the substrate by use of a shadow frame as described above, which keeps the glass edges flat and helps trap the cooling gas in the space between the substrate and the support member. However, as noted previously, conventional shadow frames typically cover a substantial portion of the substrate edge resulting in reduced usable area on the substrate. Further, the clamping force is often excessive causing damage to the substrate. For example, U.S. Pat. No. 5,753,133 provides an apparatus using a spring-biased clamping mechanism to position a substrate in a vertical position during processing. The pressure supplied by such an arrangement can cause the substrate to fracture or warp during the elevated processing temperatures.
Yet another problem with flat panel display processing is the improper positioning of the substrate on the support member and relative positioning with the shadow frame. For uniform deposition, alignment of the substrate relative to the support member as well as the shadow frame is required. While the alignment of the substrate to the support member is typically accomplished by calibration of the substrate transfer robot, which is used to position the substrate on, and remove the substrate from, the support member, the proper alignment of the shadow frame is not provided for in conventional designs. If the shadow frame is not properly centered with respect to the substrate during processing, the amount of shadowing that occurs on each edge of the substrate will be unequal. Misalignment of the shadow frame can lead to decreases in productivity and heightened operating costs.
Therefore, there is a need for a clamping and aligning assembly that minimizes edge exclusion while providing sufficient clamping force to stabilize a substrate during processing. Further, there is a need for an assembly that inhibits the drainage of plasma to chamber components.
The present invention generally provides a clamping and aligning assembly for use in a vacuum chamber. In one embodiment a shadow frame is provided having a plurality of tabs are disposed on the shadow frame and extend inwardly therefrom. The tabs comprise protruding contact surfaces for stabilizing a substrate during processing.
In another embodiment, a substrate clamping and aligning assembly is provided having a shadow frame and a floating plasma shield supported by a plurality of insulating members. A plurality of tabs are disposed on the shadow frame and extend inwardly therefrom. The tabs comprise protruding contact surfaces for stabilizing a substrate during processing. The floating plasma shield is disposed on the insulating members at a perimeter portion of a movable support member.
In yet another embodiment, a substrate clamping and aligning assembly is provided having a shadow frame, a floating plasma shield and a plurality of insulating alignment pins. A plurality of tabs are disposed on the shadow frame and extend inwardly therefrom. The tabs comprise protruding contact surfaces for stabilizing a substrate during processing. The insulating alignment pins are disposed at a perimeter of a movable support member. A tapered surface of the insulating alignment pins cooperates with a corresponding, conforming surface of an alignment recess formed in the shadow frame to urge the shadow frame into a desired position. Preferably, the floating plasma shield is disposed on the insulating alignment pins in spaced relationship to the support member and below the shadow frame to shield the perimeter of the support member during processing.
In still another embodiment, a substrate processing chamber is provided having a clamping and aligning assembly disposed therein. The processing chamber defines a processing region and includes a support member selectively movable into the processing region. The clamping and aligning assembly includes a shadow frame, a floating plasma shield and a plurality of insulating alignment pins. The shadow frame is disposed in the processing chamber adjacent to the processing region and is shaped to accommodate a substrate. A plurality of tabs are disposed on the shadow frame and extend inwardly therefrom. The tabs comprise protruding contact surfaces for stabilizing a substrate during processing under the weight of the shadow frame. The insulating alignment pins are disposed at a perimeter of the support member and include an upper tapered surface that cooperates with a corresponding, conforming surface of an alignment recess formed in the shadow frame to urge the shadow frame into a desired position during the upward motion of the insulating alignment pins. Preferably, the floating plasma shield is disposed on the insulating alignment pins in spaced relationship to the support member and below the shadow frame to shield the perimeter of the support member during processing.